Thermoplastic resins, which have good electrical insulation characteristics, are widely used to provide insulating jacketing or sheaths for wire and cable. Recently, there has been a demand for improved flame retardant properties, e.g., as high as V-1 to V-0 using Underwriters Laboratories standards.
Thermoplastic resin can be made flame retardant by adding to the resin organic halides or antimony oxides, for example, or the resin itself can be halogenated such as polyvinyl chloride or chlorinated polyethylene. These thermoplastic resins, however, on burning, sag and emit large amounts of smoke and other harmful gases, and also corrode metals.
In order to solve these problems, it has been proposed to add metal hydroxides to non-halogenated thermoplastic resins. Aluminum hydroxide was first used because of its low cost, but it has such a low decomposition temperature (170.degree. to 190.degree. C.) that the aluminum hydroxide decomposes generating water, which, in turn, causes foaming on interior surfaces. Furthermore, to obtain a flame retardance of V-1 to V-0, aluminum hydroxide has to be added to the thermoplastic resin in large amounts, e.g.. 60 to 65 percent by weight based on the weight of the total composition.
Compared with aluminum hydroxide, magnesium hydroxide has a much higher decomposition temperature (about 360.degree. C.), and, thus, exhibits less foaming. For this reason, and others, magnesium hydroxide has been widely used as a flame retardant in resins. One disadvantage of a thermoplastic resin/magnesium hydroxide flame retardant composition, however, is that when it is applied to an electrical conductor as an outer jacket, the jacket exhibits blushing when it rubs against another jacket or other hard objects, is scratched, or bent.
Blushing or "stress whitening" occurs when a resin, in jacket or molded form, is stressed or impacted and results in a white mark on the surface of the resin at the point of impact. These white marks not only detract from the appearance of the jacket, but also have a degrading effect insofar as arc resistance, insulation, mechanical, and other properties are concerned.